The aim of this study is to collect and analyze data on the social organization of sexual behavior, particularly the social structuring of sexual action, and the ways in which that structuring influences behaviors that increase the incidence and prevalence of a variety of health-related problems. Informed by the social network and lifecourse perspectives, it will address the following issues: 1) the incidence and prevalence of particular sexual activities across an adult population; 2) the way the social characteristics of individuals structure their likelihood of forming sexual relationships with specific others; 3) the organization of the behaviors constituting sexual transactions; 4) the organization of sexual partnering and behavior across the lifecourse; and 5) the ways partners negotiate, routinize, and understand their sexual transactions. Data will be collected via a complex, high quality face-to-face survey with approximately 2,500 adults, aged 18 to 44 years (the most sexually active part of the population), drawn from households in two middle-sized metropolitan areas using a three stage area probability sample design. The interview includes items on specific sexual activities, attitudes and beliefs about sex, and sequences of partnership formation and dissolution, as well as background information, such as age, socioeconomic status, and ethnic and religious group. The basic analytic strategy is to identify the full range of sexual behaviors and to examine patterns that may be associated with specific types of partnerships and/or specific attitudes. The network structure of partnership formation, particularly with regard to variables such as race, socio-economic status, age, and religious background will also be modeled using recent advances in network analysis. The relative size of population subgroups from the two semi-autonomous communities will serve as a baseline from which to measure the nonrandom component of partner selection. Event-history analyses will be used to determine the conditions under which people enter and withdraw from the network of sexual access.